In a heterodyne receiver, a received signal is shifted to a much lower fixed intermediate frequency (IF) while leaving information modulated on the carrier signal unaltered. The lower frequency signal is easier to process. The required frequency translation to the intermediate frequency is usually accomplished by mixing the received signal with a locally generated signal which differs from the received signal by the intermediate frequency.
In a typical heterodyne receiver, there is at least one band-pass filter at the IF stage designed to reject signals in adjacent channels. Performance requirements imposed on the receiver, or insufficient rejection ratio of a single band-pass filter, may require additional band-pass filter in order to adequately reduce interference from adjacent channels. These band-pass filters can have significant insertion loss depending on their design
Heterodyne receivers are usually designed with sufficient filtering to satisfy the required performance specifications while operating under worst adjacent channel interference conditions, even though such interference is not always present. Multiple filters, when uncompensated, can attenuate or otherwise degrade the received signal. This is especially true when a series of filters are cascaded, because insertion loss is additive, see U.S. Pat. No. 6,112,069 “Radio receiver and method for suppressing attenuation properties of a low frequency signal,” issued to Na on Aug. 29, 2000.